It is known in the art to use inks, compositions, coatings or layers containing oriented magnetic or magnetizable pigment particles, particularly also optically variable magnetic or magnetizable pigment particles, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetizable pigment particles are disclosed for example in U.S. Pat. Nos. 2,570,856; 3,676,273; 3,791,864; 5,630,877 and 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features on the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the principle that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile sense while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature.
Magnetic or magnetizable pigment particles in printing inks or coatings allow for the production of magnetically induced images, designs and/or patterns through the application of a correspondingly structured magnetic field, inducing a local orientation of the magnetic or magnetizable pigment particles in the not yet hardened (i.e. wet) coating, followed by the hardening of the coating. The result is a fixed and stable magnetically induced image, design or pattern. Materials and technologies for the orientation of magnetic or magnetizable pigment particles in coating compositions have been disclosed for example in U.S. Pat. Nos. 2,418,479; 2,570,856; 3,791,864, DE 2006848-A, U.S. Pat. Nos. 3,676,273, 5,364,689, 6,103,361, EP 0 406 667 B1; US 2002/0160194; US 2004/0009308; EP 0 710 508 A1; WO 2002/09002 A2; WO 2003/000801 A2; WO 2005/002866 A1; WO 2006/061301 A1. In such a way, magnetically induced patterns which are highly resistant to counterfeit can be produced. The security element in question can only be produced by having access to both, the magnetic or magnetizable pigment particles or the corresponding ink, and the particular technology employed to print said ink and to orient said pigment in the printed ink.
EP 1 641 624 B1, EP 1 937 415 B1 and EP 2 155 498 B1 disclose devices and method for magnetically transferring indicia into a not yet hardened (i.e. wet) coating composition comprising magnetic or magnetizable pigment particles so as to form optical effect layers (OELs). The disclosed methods advantageously allow the production of security documents and articles having a customer-specific magnetic design.
EP 1 641 624 B1 discloses a device for magnetically transferring indicia corresponding to the design to be transferred into a wet coating composition comprising magnetic or magnetizable particles on a substrate. The disclosed device comprises a body of permanent-magnetic material being permanently magnetized in a direction substantially perpendicular to the surface of said body, wherein the surface of said body carries indicia in the form of engravings, causing perturbations of its magnetic field. The disclosed devices are well suited for transferring high-resolution patterns in high-speed printing processes such as those used in the field of security printing. However, and as described in EP 1 937 415 B1, the devices disclosed in EP 1 641 624 B1 may result in poorly reflecting optical effect layers having a rather dark visual appearance. The disclosed drawback of EP 1 641 624 B1 results from the mainly perpendicular orientation of the magnetic pigment particles with respect to the printed substrate plane over a large part of the oriented coating layer, as resulting from the perpendicular magnetization which is required in said device.
EP 1 937 415 B1 discloses an improved device for magnetically transferring indicia into a wet coating composition comprising magnetic or magnetizable pigment flakes on a substrate. The disclosed device comprises at least one magnetized magnetic plate having a first magnetic field and having surface relief, engravings or cut-outs on a surface thereof representing said indicia and at least one additional magnet having a second magnetic field, wherein the additional magnet is fixedly positioned adjacent to the magnetic plate so as to produce substantial overlap of their magnetic fields. The presence of the at least one additional magnet has the effect of flattening out the magnetic field lines generated by the at least one magnetized permanent-magnetic plate, resulting in a more appealing visual effect. While the disclosed device flattens out the magnetic field lines compared to prior art, the field lines remain essentially curved. The disclosed device may still lead to the undesirable appearance of large dark areas in the magnetically transferred image, in particular in zones where the magnetic field lines are substantially perpendicular to the substrate surface. EP 1 937 415 B1 does not teach how to produce an even distribution of pigment flake orientations that would result in strongly reflecting OEL that are particularly well suited to carry customer specific indicia.
The methods and devices described hereabove use magnetic assemblies to mono-axially orient magnetic pigment particles. Mono-axial orientation of magnetic pigment particles result in neighboring particles having their main (second longest) axis parallel to each other and to the magnetic field, while their minor axis in the plane of the pigment particles is not, or much less constrained by the applied magnetic field. Accordingly, a sole mono-axial orientation of magnetic pigment particles results in optical effect layers that may suffer from a low reflectivity and brightness as light is reflected in a wide range of directions, especially in directions that are substantially perpendicular to the magnetic field lines.
EP 2 155 498 B1 discloses a device for magnetically transferring indicia into a coating composition comprising magnetic or magnetizable particles on a substrate. The disclosed device comprises a body subjected to a magnetic field generated by electromagnetic means or permanent magnets, which body carries determined indicia in the form of engravings on a surface of the body. The disclosed body comprises at least one layer of material of high magnetic permeability in which said engravings are formed and wherein, in un-engraved regions of said layer of material of high magnetic permeability, the field lines of the magnetic field extend substantially parallel to the surface of said body inside the layer of material of high magnetic permeability. It is further disclosed that the device comprises a base plate of material of low magnetic permeability supporting the layer of material of high magnetic permeability, wherein said layer of material of high magnetic permeability is preferably deposited on the base plate by galvanization. EP 2 155 498 B1 further discloses that the main direction of the magnetic field lines may be changed during exposure of the layer comprising magnetic or magnetizable particles by rotating, advantageously by 360°, the magnetic field. In particular, EP 2 155 498 B1 discloses embodiments wherein permanent magnets are used instead of electromagnets and wherein the rotation of said permanent magnets may be performed by physical rotation of the magnets themselves. A drawback of the disclosed devices resides in the galvanization process since said process is cumbersome and needs special equipments. Moreover, a significant shortcoming of the disclosed invention is that the process relies on the physical rotation of the permanent magnets to achieve 360° rotation of the magnetic field. This is particularly cumbersome from an industrial point of view as it requires complex mechanical systems. Furthermore, rotating simple magnets as suggested produces essentially spherical pigment flake orientations as shown in the corresponding examples of EP 2 155 498 B1. Such orientations are not well suited to clearly reveal indicia with an eye-catching relief/3D effect, as the sphere-like effect is superimposed with the indicia. The only method that can be derived from the description to generate relatively flat rotating fields would be to rotate very large magnets, which is impractical. EP 2 155 498 B1 does not teach how to establish a practical industrial process to generate rotating magnetic fields that impart an appealing 3D/relief impression of the indicia.
WO 2015/086257 A1 discloses an improved method for producing an optical effect layer (OEL) on a substrate, said process comprising two magnetic orientation steps, said steps consisting of i) exposing a coating composition comprising platelet-shaped magnetic or magnetisable pigment particles to a dynamic, i.e. direction changing, magnetic field of a first magnetic-field-generating device so as to bi-axially orient at least a part of the platelet-shaped magnetic or magnetisable pigment particles and ii) exposing the coating composition to a static magnetic field of a second magnetic-field-generating device, thereby mono-axially re-orienting at least a part of the platelet-shaped magnetic or magnetisable pigment particles according to a design transferred by said second magnetic-field-generating device. WO 2015/086257 A1 provides an example where the second magnetic orientation step uses a second magnetic-field-generating device such as those described in EP 1 937 415 B1. Whereas the method disclosed in WO 2015/086257 A1 allows the production of optical effects layers exhibiting improved brightness and contrast compared to the prior art, the so-obtained optical effects layers may still suffer from a poorly reflecting visual appearance and does not teach how to impart an appealing 3D/relief impression to the indicia.
Therefore, a need remains for improved processes for magnetically transferring indicia so as to produce optical effect layers (OELs) exhibiting better reflecting visual appearance, wherein said processes should be reliable, easy to implement and able to work at a high production speed while allowing the production of OELs exhibiting not only an eye-catching relief and/or 3D effect but also a bright and well resolved appearance.